Aluminum alloy



Patented Mar. 30, 1937 PATENT. OFF/ICE ALUMINUM ALLOY John G. G. Frostand Walter Bonsack, Cleveland,

Ohio, assignors to The National smelting Company, Cleveland, Ohio, acorporation of Ohio No Drawing.

Application October 4, 1935, Serial 8 Claims. (01. 15-147) Thisinvention relates to alloys, and more particularly aluminum base alloyshaving a low thermal expansion and high thermal conductivity.

In the making of cast articles, such as pistons,

cylinder heads and similar articles utilized in internal combustionengines and motor constructions, it is desirable to make such articlesfrom aluminum and alloys thereof because of their 10 low specificgravity.

While aluminum and aluminum alloys in general have a high thermalexpansion, some aluminum alloys have been developed with a considerablylower thermal expansion, making them more desirable for use in themanufacture of castings for motor parts.

It has been found, however, that, in addition to having a low thermalexpansion, aluminum alloys for use in making motor part castings shouldalso have a high thermal conductivity, as well as good mechanical andphysical properties, such as hardness, machinability and suitablestrength at the elevated temperatures occurring in internal combustionengines.

It is, therefore, an object of this invention to provide a light weightaluminum base alloy having a comparatively low thermal expansion and acomparatively high thermal conductivity, and also having suitablemechanical and physical properties for the casting of motor parts andother similar articles requiring similar properties.

Another object of this invention is to provide such an aluminum basealloy which is readily machinable, which may be readily cast in the 5usual type of molds, and which has a fine, ho-

mogeneous grain structure, and which may be subjected to elevatedtemperatures without causing grain growth or loss of strength.

While certain aluminum-silicon alloys are known to have a relatively lowthermal expansion and relatively high thermal conductivity as comparedto other aluminum alloys, it is necessary with these aluminum-siliconalloys to have the other desirable properties of hardness, machinabilityand good bearing qualities to make them more suitable for the productionof pistons and other motor parts, and it is an object of this inventionto provide such an aluminum-silicon alloy with such' desirablemechanical properties and still maintain the relatively low thermalexpansion and relatively high thermal conductivity of these alloys.

The addition of magnesium to aluminum-silicon alloys increases theirhardness and tensile 55 strength, and increases their elasticproperties,

and, in the amount desirable to give these properties, has little or noeffect on the low thermal expansion and high thermal conductivity.

It has been discovered that if tin is added in proper proportions toaluminum base alloys containing silicon it will increase the thermalconductivity without materially affecting the thermal expansion of thealloy. Tin also improves the machinability of the alloy, and providesbetter bearing qualities in castings made therefrom. 10

excellent properties for the manufacture of ar- 15' ticles of thisnature.

In preparing our improved aluminum base alloy, silicon is used as thepredominating ingredient and may be present in amounts ranging from 7%to 20%. Proportions of silicon greater than 7% are more effective inreducing the thermal expansion of the alloy. It is desirable, however,to avoid the use of too large a proportion of silicon, since it has atendency to segregate out in large crystals, unless the casting ischilled very rapidly. It is, therefore, preferable to use silicon in anamount from 11% to 15%.

Tim is an important addition to the aluminumsilicon alloy in order toprovide better thermal conductivity and bearing qualities, as well as toimprove its machinability. In order to obtain these properties, tin maybe present in the amount of .3% to 3%, and preferably in an amount from.5% to 1.5% or 2%.

Many types of castings where rapid heat transfer is desired, such as forcompressors, motor parts and other purposes where the castings aresubjected to elevated temperatures, or even for refrigerator parts andother uses where the castings are used at low temperatures, can beadvantageously made of an alloy of aluminum, silicon and tin, withoutmagnesium.

In order to increase the hardness of the alloy when it is subjected toheat treatment, and to produce an alloy having desirable machinablequalities, it has been found that the addition of magnesium isadvantageous. This may be in an amount from approximately .1% to 2%, and

preferably magnesium is utilized in an amount up to 1%, such assubstantially .5% -to .9%, as as excess of magnesium has a tendency torender the alloy brittle, and a tendency to reduce the thermalconductivity of the alloy to too great a degree.

When present in small amounts up to apmal conductivi proximately 3%,zinc has the eflect of improving the mechanical properties such asincreasing the tensile strength and hardness of the metal. When added inamounts such as 2% to 3% it tends to reduce the thermal conductivity ofthe alloy and to increase the thermal expansion. If relatively highthermal conductivity and relatively low thermal expansion are desired,it is preferable to have zinc present in amounts less than 2%, such as.5% to 1.5%, and when present in these amounts the thermal conductivityand thermal expansion of the alloy are not materially aflected.

Copper may be present in the alloy in small amounts, for in amounts suchas .6% or less it does not seem to materially afiect the thermalexpansion of the alloy, and it is, therefore, not

necessary, in making the alloy, to exclude base metals containing smallamounts of copper. If copper is present in the alloy in the amount ofapproximately 1% or more, it has the effect of,

substantially reducing the thermal conductivity of the alloy.

If relatively high thermal conductivity of the alloy is desired, coppershould not be present in an amount more than approximately 2%, but whensuch a high degree of thermal conductivity is not necessary, the coppermay be present in amounts up to about .6% or more. I

Chromium, manganese, nickel, cobalt and titanium tend to reduce thethermal conductivity of aluminum-silicon alloys. These metals also havea tendency to form compounds with aluminum at a high melting point, andsuch compounds tend to segregate from the molten metal beforesolidification occurs. When a relatively high thermal conductivity ofthe alloy is desired, the above metals should not be present in anamount suflicient to materially reduce the thermal conductivity, and, ifpresent at all, should only be present in such minimum amounts as arepracticable in the commercial manufacture of the alloy, or as will notreduce the thermal conductivity beyond the limit desired for the alloy.Iron also has the tendency to reduce therbut iron, as is well known, isusually present an impurity in aluminum or aluminum alloys. However, itshould not be present in an excessive amount, such as 1% or more.

As illustrations of our improved alloys the following specific examplesare given:

An alloy containing 13% silicon, 1% tin, .5% magnesium and approximately.1% copper, and the balance aluminum and inner impin'ities, was chillcast and aged for 15 hours at 200 C. Upon being tested it was found tohave a thermal expansion of 19.9 10 per degree centigrade between atemperature range of 20 and C. It also had a thermal conductivity of .38calories per square centimeter per second at 30 C.

In an alloy having the same composition as above, except that 1%'copperwas included instead of approximately .1% copper, the coeflicient ofthermal expansion was substantially the same, while the thermalconductivity was approximately .36 calories per square centimeter persecond at 30 C. after being aged at 20050.

For some purposes, where a high degree of' machinability is not soessential, the quantity of magnesium in the alloy may be substantiallyreduced or practically eliminated, and the alloy has a low coefficientof thermal expansion and high thermal conductivity. An example of suchan alloy is an alloy containing 13% silicon, 1%

tin and the balance aluminum and nnnor impurities, which was chill castand aged for 15 hours at 200 C. Upon being tested it was found to have athermal expansion of approximately 20 10- per degree centigrade betweena temperature range of 20 and 100 C., and it also had thermalconductivity of .37 calories per square centimeter per second at 30 C.With the same alloy, with the exception that the amount of tin was 2%,instead of 1%, the thermal conductivity was approximately .38 caloriesper square centimeter per second at 30 C., and the thermal expansion wasthe same.

In an alloy containing 13% silicon, 1% tin and .5% magnesium thecoejicient of thermal expansion was substantially the same as that ofthe alloys given above, and the thermal conductivity was approximately.37 calories per square centimeter per second at 30 C., whereas when theamount of magnesium was increased to 1% in this alloy, the silicon andtin content being the same, the thermal conductivity was approximately.33 calories per square centimeter per second at 30 C.

As previously stated our invention contemplates that copper may bepresent in small amounts in the alloy and it is also to be understoodthat minor impurities may be present without departing from the scope ofthe invention set forth in the claims.

Low thermal expansion and high thermal conductivity alloys containingaluminum, silicon and tin are described and claimed in our copendingapplication, Serial No. 96,517, filed August 17, 1936; aluminum basealloys having similar characteristics and containing aluminum, silicon,tin and zinc are described and claimed in our copend ing application,Serial No. 96,519, filed August 17, 1936; and aluminum base alloys of asimilar character, containing aluminum, silicon, tin, magnesium and'zinc, are described and claimed in our copending application, SerialNo. 96,518, filed August 17, 1936.

Furthermore, it will be understood that the present invention is notlimited to the specific details set forth in the foregoing examples,which should be construed as illustrative, and not by way of limitation,and in view of the numerous modifications which may be effected thereinwith out departing from the spirit and scope of this invention, it isdesired that only such limitations be imposed as are indicated in theappended claims.

What we claim is:

1. A machinable aluminum base alloy having a low coeflicient of thermalexpansion and high thermal conductivity, comprising about 11 to 15%silicon, about .5 to 2% tin, about .5 to .9% magnesium, with the balancesubstantially all aluminum.

2. A machinable aluminum base alloy having a low coefiicient of thermalexpansion and high thermal conductivity comprising 7 to 20% silicon, .3to 3% tin and .1 to 2% magnesium with the balance substantially allaluminum.

3. An aluminum base alloy having a low coemcient of thermal expansionand high thermal conductivity comprising about 11 to 15% silicon. .3 to3% tin and .1 to 2% magnesium with the balance substantially allaluminum.

4. An aluminum base alloy having a low coefllcient of thermal expansionand high thermal conductivity, comprising about 11 to 15% silicon, about.5 to 2% tin, .1 to 2% magnesium, with the balance substantially allaluminum.

5. An aluminum base alloy having a low coefllcient of thermal expansionand high thermal conductivity, comprising about 11 to 15% silicon, about.5 to 1.5% tin, .1 to 1% magnesium with the balance substantially allaluminum.

6. An aluminum base alloy having a low coemcient of thermal expansionand high thermal conductivity, comprising about 11 to 15% silicon, about.5 to 2% tin, about .5 to 1% magnesium,

with the balance substantially all aluminum, said alloy having acoemcient of thermal expansion as cast of approximately 20x 10-, orless, per degree centigrade between a temperature range of 20 and 100 C.and having a thermal conductivity oi approximately .33 calories or moreper square centimeter per second at 30 C.

"l. A piston formed from an aluminum base al- 10y having a lowcoeflicient of thermal expansion and high thermal conductivitycomprising about 11 to 15% silicon, .1 to 1% magnesium, about .5 to 1.5%tin with the balance substantially all aluminum.

8. A chill casting formed from an aluminum base alloy having a lowcoemcient or thermal expansion and high thermal conductivity, comprisingabout. 11 to 15% silicon, .5 to 3% tin and .5 to 1% magnesium, with thebalance substantially all aluminum.

JOHN G. a. mom. WALTER aonsacx CERTIFICATE OF CORRECTION.

Patent No. 2,075,517. March 30, 1937.

JOHN G. G. FROST, ET AL.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,first column, line 55, for the word "inner" read minor; and that thesaid Letters Patent should be read with this correction therein that thesame may conform to the record of the case in the Patent Office.

Signed and sealed this 1st day of June, A. D. 1957.

Henry Van Arsdale (Seal) Acting Commissioner of Patents.

